Parts for 7 traps (4 corner + 3 basic) cost about 300 euros. It took about 30 hours to build these but alot more time to search information about the basstraps, design them and search places where you can buy the right materials. It can also take alot more time if you don't have much experience in such a work. You also need several tools and a place where you can build the traps.
If I had ordered the same kind of basstraps from USA, it would have cost me over 1000 euros including shipping. IMO nobody sells good basstraps in Europe. Almost all traps which are available here are made of foam which doesn't work as well at low frequencies and the ones which are not made of foam are overpriced and small. The only good basstraps which I found are made by Realtraps and GIK Acoustics in USA.
So, IMO it was worth it altough I had to spend over half of my summer holiday planning and building them. The other route was simply too expensive for me and I really wanted to have basstraps. I am very satisfied with how they look and I can barely wait to hear my new treated room in September.
There are alot of good articles about building your own basstraps in the internet, just google them. However, I can say 2 things which are the most important imo:
1) Use wool which has a density of at least 48kg/m^3
2) Seal the traps carefully that no wool dust or fibers can enter the room air where you're working. A simple fabric coating may not be enough alone.
3) If you live in USA, buy your traps from GIK if you don't want to spend too much money.